System.Waf

• WAF: Change ValidatableModel to match the INotifyDataErrorInfo.GetErrors description. [Breaking change]
• WAF: ThrottledAction: Improve performance of InvokeAccumulated.
• WAF: Remove the FileType.FileExtension limitation that it has to start with a '.' point character.
• WAF: Change FileType to expose FileExtensions collection instead of FileExtension with the concatenated string. [Breaking change]
• WAF: Add NullToVisibilityConverter class.
• WAF: Add ResourceHelper class.
• Samples: Improve WPF design time support.
• InfoMan: Improve validation logic (Replace custom ValidationModel class with ValidatableModel).
• BookLibrary: Improve validation error style.